Organic light emitting diodes (OLED) become red-hot flat panel display products worldwide in recent years. That is because an OLED display has characteristics such as self-luminous, a wide view, a short response time, a high light-emitting efficiency, a wide color gamut, a low working voltage, a thin panel, being able to be produced with a large size and a flexible panel, and a simple manufacturing process, etc. In addition, the OLED display has potential to be further developed with a low cost.